“They say the military option is on the table but it’s not being viewed as credible,” Ryan said during the Danville, Ky., debate.

The Wisconsin lawmaker said Iranian leaders likely question Obama’s determination when they see him at odds with the Israeli prime minister over the issue, and when top Defense Department officials “walk” back White House suggestions that the United States would take military action against Tehran, if necessary.

Vice President Joe Biden gave no quarter, though, touting what he called a “crippling” four rounds of U.N. Security Council economic sanctions against Tehran and defending the president’s credibility. He called the Republican ticket's criticism of his administration’s Iran policy “bluster.”

“The ayatollah sees his economy being crippled. The ayatollah sees that there are 50 percent fewer exports of oil,” said the vice president. “He sees the currency going into the tank. He sees the economy going into free fall. And he sees the world for the first time totally united in opposition to him getting a nuclear weapon.”

Biden also assured that neither Obama nor any U.S. president would deceive when it comes to threatening military action.

In a speech before the U.N. General Assembly late last month, Obama signaled that he remains committed to diplomatic, economic, and political tools in addressing the Iran issue, but also would explore the full range of options if Tehran does not bend.

“America wants to resolve this issue through diplomacy, and we believe that there is still time and space to do so. But that time is not unlimited,” the president said. “The United States will do what we must to prevent Iran from obtaining a nuclear weapon.”

At Thursday’s debate, Ryan asserted that Iran could be peacefully prevented from developing an atomic bomb only if “the ayatollahs change their minds,” suggesting that there is little chance of that occurring under a second Obama-Biden term. Tehran’s nuclear developmental program -- which Iranian leaders insist is entirely peaceful -- has continued apace, said the lawmaker.

“This administration has no credibility on this issue,” said Ryan.

He also charged that the White House had “watered down” and “delayed” sanctions. Ryan said he and other Republicans had pushed for stronger sanctions against Iran since 2007, but as of 2009 “the administration was blocking us every step of the way.”

“Imagine had we let the Republican Congress work out the sanctions,” Biden shot back. “Do you think there’s any possibility the entire world would have joined us? Russia and China? All of our allies?

“These are the most crippling sanctions in the history of sanctions,” he said, noting that Republican presidential nominee Mitt Romney had voiced support for the international economic penalties.

“When you’re talking about doing more, what are you -- are you going to go to war?” asked Biden, sounding incredulous. “Is that what you want to do?”

Throughout the back-and-forth over Iran, Ryan sought to portray the administration as weak and lacking resolve, while Biden -- at some points on the defensive -- painted Romney and Ryan as naïve, and bordering on reckless, when it comes to foreign policy.

Addressing a question from Martha Raddatz of ABC News, the debate moderator, Biden said a military strike against Iranian facilities -- if ultimately necessary -- could make headway in weakening any effort to build an atomic warhead.

“We feel quite confident we could deal a serious blow to the Iranians,” the vice president said.

Ryan asserted that Iran already had enough fissile material to make five bombs, up from a cache sizable for just one nuclear weapon when Obama took office in 2009. However, Biden -- a longtime foreign policy leader in the Senate prior to the 2008 elections -- took issue with Ryan’s portrayal of how close Iran is to becoming a nuclear power.

“There is no weapon that the Iranians have at this point,” Biden said, noting that Iran’s 20 percent uranium enrichment level remains well short of the 90 percent purity required for a bomb, and that building a warhead would require additional time. “We'll know if they've started the process of building a weapon,” he said.

The U.S. and Israeli intelligence communities now agree that Iran remains “a good way away” from “getting a nuclear weapon,” Biden said. “There is no difference between our view and theirs.”

Israeli Prime Minister Benjamin Netanyahu has pushed Obama to set a "red line" for military action against Tehran, but late last month said it would likely be next spring at the earliest before Iran could enrich uranium to weapon-grade level. Iran this week attempted to ease global concerns by announcing it was converting more than one-third of its processed uranium into a powdered form that would be largely unsuitable for a weapon.

Ryan held that a nuclear-armed Iran would have worse ramifications than a preemptive military strike against that nation, despite a warning last week by former Defense Secretary Robert Gates.

"The results of an American or Israeli military strike on Iran could, in my view, prove catastrophic, haunting us for generations in that part of the world,” Gates said in a Norfolk, Va., appearance. "Such an attack would make a nuclear-armed Iran inevitable. They would just bury the program deeper and make it more covert."

Ryan voiced a different view.

“A nuclear-armed Iran, which triggers a nuclear arms race in the Middle East” is the worst possible outcome, he said on Thursday. “This is the world's largest sponsor of terrorism.”

Ryan maintained that if Iran gets “nuclear weapons, other people in the neighborhood will pursue their nuclear weapons, as well.”

Biden took no issue with Ryan’s depiction of the risks and possible necessity of a military strike against Iran, but underlined that “war should always be the absolute last resort.”